Field of the Invention
This invention relates generally to ophthalmic lenses, and more specifically to a multifocal ophthalmic lens having an optical add power progression, as well as a system and method for providing such a multifocal ophthalmic lens.
Description of the Background
An ophthalmic lens, such as an intraocular lens (IOL), a phakic IOL, or a corneal implant, by way of non-limiting example, may be used to enhance patient vision. For example, an IOL may be used to replace the natural lens of an eye that is removed during cataract surgery.
Multifocal lenses, including multifocal IOLs, may replace the function of the eye's natural lens in providing multifocal vision, such as by dividing incident light to two different focal points to provide both near and distance vision. The use of diffractive or refractive optics in ophthalmic lenses, in order to provide multifocal/bifocal division of incident light, is well understood to those skilled in the pertinent arts.
In a bifocal IOL, the optic area may be divided into a plurality of annular zones that are offset parallel to the optical axis to provide a specific diffractive or refractive relationship between the zones. As used herein, “annular” is defined to be ring-shaped, substantially ring-shaped, or at least partially ring-shaped. The annular zones may form a concentrically arranged pattern characterized by the optical power step between zones, the circumferential spacing between zones, and the surface profile of each zone. These concentric annular zones are typically configured to maintain a predefined relationship of light passing through the zones to effect bifocal vision. Unfortunately, the abrupt optical power step between zones makes it difficult for such a solution to provide improved far and near vision simultaneously, while avoiding glare, halos, decreased contrast sensitivity, and increased pupil dependence. Further, such solutions are not designed to achieve intermediate vision, which would optimally correct presbyopia.
In known alternative embodiments of a multifocal lens, the zones may be non-radially symmetric (see the Lentis MPlus® lens by OcuLentis), such as in the embodiment illustrated in FIG. 1 as 23b. More particularly, center and upper zones may be at least substantially circular and semi-circular respectively, and may have thereabout below a portion having a different optical power than the center and upper zone. In such a lens, the upper and central portion of the optic may be used for distance vision, and the optical add power may be constrained to the lower portion of the lens, in the non-radially symmetric distribution, as would be the case for a bifocal spectacle lens. As used herein, the term “non-radially symmetric” is used to indicate that the distribution is not the same for all the points at the same distance from the center of the optic in the lens plane.
As such, the zones of these exemplary bifocal IOLs may form a bifocal lens that may, for example, produce a first focal point for distant vision, and a second focal point corresponding to near distances. A preferred characteristic of lenses that incorporate diffractive/refractive zones in this manner is that the amount of light in the near and distant foci be substantially constant for all pupil sizes. However, it might be desired in certain instances to increase the amount of light in the distant focus as the pupil size increases, for instance under intermediate or low light conditions. One way to increase the amount of light dedicated to distance vision is to restrict the zone producing the second focal point to the central portion of the lens, and to make the outer region of the lens refractive only.
A particular disadvantage associated with the radially symmetric-type of bifocal IOL is the aforementioned problem of halos. In the case of halos, light from the unused foci creates an out-of-focus image that is superimposed on the used foci, in part due to the abrupt change in optical power between adjacent ones of the annular zones. For example, if light from a distant point source is imaged onto the retina of the eye by the distant focus produced by a concentric bifocal IOL, the near focus produced by that IOL may simultaneously superimpose a defocused image on top of the image formed by the distant focus, thus creating a halo effect. Thus, it is not possible to have high contrast images either at the far focus or near focus. Although non-radially symmetric IOLs may partially address halos, such halos are constrained to the part of the field of view where the added power is addressed. In addition, the abrupt optical power changes between zones preclude complete elimination of halos and glare.
Therefore, the need exists for a lens, and a system and method of providing a lens having an optical add power progression that allows for the following: far, near and intermediate vision with good visual performance at all distances; significantly reduced halos and glare; improved contrast sensitivity; and minimized pupil dependence.